The objective of the Phase I proposal is to assess the accuracy of a non-invasive, isothermal rebreathing method for measuring the mixed venous partial pressure of highly soluble gases. This approach will allow the semi-invasive (without a pulmonary catheter) measurement of the ventilation/perfusion (V-A/Q) distribution of the lung. During Phase I, a prototype device will be designed, fabricated, and tested on dogs. If successful, the design could lead to a lower cost and easier method of obtaining V-A/Q distributions on critically ill pulmonary patients. This would allow access to the estimated $130 million total United States V-A/Q market.